Enable viscous flux contribution at supersonic outlet boundary condition

[yiluntam]

Proposed Changes

The viscous flux contribution in BC_Supersonic_Outlet was previously commented out due to reported convergence issues. However, for viscous NS simulations this omission is physically incorrect and causes severe convergence problems at outlet boundary cells, which also prevents the use of second-order MUSCL reconstruction.

For supersonic flows, downstream disturbances cannot propagate upstream, so the outlet state can be safely extrapolated from the domain interior. Using SetPrimitive(V_domain, V_domain) instead of (V_domain, V_outlet) resolves the convergence problems that motivated the original commenting.

PR Checklist

• [ x ] I am submitting my contribution to the develop branch.
• [ x ] My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.

[yiluntam]

@pcarruscag Could you take a look at this when you have time? Thanks!

[yiluntam]

Validation: Ma = 4, Tw/T∞ = 1, laminar flat plate boundary layer. Cf distribution agrees well with Anderson's hypersonic reference ("Hypersonic and High Temperature Gas Dynamics").

As can be seen, the new code aligns more closely with the theoretical solution. The distribution of physical quantities at the outlet is also more reasonable. Furthermore, the convergence time required during the calculation has been reduced.